I've had that said to me a few times. Usually I just say, "Yes I am their REAL mom".
We have 2 bio & 3 adopted. I get asked things like, "What do your real children..." Again I just say, "They are ALL my REAL children".
Sometimes I get asked,"What does their real mother....." I say, "Well, their BIRTH Mother...."
This one I love, "Are they all related?" DUH, you idiot!!!!

I just cringe on the inside & say, "They have been since birth" or "They are now"
Get this one, OHHHH you'll love it!!!
One woman said to me, "You need to take care of your OWN children first" referring to our bio children!!!!!!!!! OH YEA! That got me going! You should have heard the rest of the conversation!!!! It started out ugly before she made that comment & it was the first time I actually got really upset & raised my voice at a comment like that.
My Mother In Law is great, too. She says she has 2 Grandchildren & 3 adopted ones.

What an idiot!! DOn't even get me started on her!!! SHe says that when she even admits she has more than 2 Grandchildren (our 2 bio)!!!!!!!
Deb